1. Field of the Invention
This invention relates to a freezing apparatus and, more particularly, to a novel improvement for increasing the freezing efficiency of a rotary compressor employed in the freezing apparatus.
2. Description of the Prior Art
Heretofore, reciprocating compressors were generally used in freezing units such as refrigerators or ice making machines. Recently, however, the use of rotary compressors has also been suggested as a way to save space in such devices.
It is generally known that, in a rotary compressor, the inside of the shell thereof is maintained at a relatively high pressure, so that the lubricating oil is charged at the high pressure side, and that, when the compressor is at a standstill, the refrigerant is mixed into the compressor oil. When the compressor is started with the refrigerant thus mixed into the oil, the refrigerant remains in the oil since the pressure in the oil chamber of the compressor is not lowered contrary to conventional reciprocating compressors. The result is that a shortage in the amount of refrigerant circulated through the refrigerant system occurs and increases in pressure at the high pressure side are not achieved smoothly. The effect of the shortage of refrigerant is more pronounced in a compressor charged with only a limited amount of refrigerant. Thus, since it is difficult to increase the pressure at the high pressure side in this manner, the pressure at the low pressure side becomes extremely low and a vacuum running phenomenon occurs in which the amount of the refrigerant in circulation is significantly reduced.
One of the reasons why the pressure increase at the high pressure side of the compressor is retarded is that the compressor at ambient temperature acts to cool the refrigerant due to its large thermal capacity and because heat radiation from the shell surface is also considerable due to the fact that the higher pressure side is in the compressor shell.
As a result of the above described phenomenon, the cooling properties at the beginning of the compressor starting operation will be lowered especially at lower ambient temperatures. This causes a refrigerating or ice making operation to be continued for an extended time, so that energy loss is correspondingly increased. For example, when the ice making machine performs the ice making operation in the above-described condition, the shape of the ice product(s) formed on the freezing mold at the inlet side of the evaporator may differ considerably from that of the ice products at the outlet side of the evaporator. The reason for this is that the pressure at the low pressure side becomes unusually low from the time the ice starts, the temperature at the inlet of the evaporator or the vaporization temperature being extremely low, and the temperature at the outlet side not being significantly lowered, thus resulting in an undesirable temperature balance between the inlet and the outlet of the evaporator.
The above-described phenomenon will become more pronounced when a lower ambient temperature prevails about the ice making machine. However, the same phenomenon occurs from time to time at a room temperature of about 10.degree. C., which temperature is in excess of the sensing temperature (4.degree. C. to 8.degree. C.) of the defrosting or harvesting completion sensor thermostat so far used as means for sensing the completion of the harvesting cycle. The above-described problem, due to the undesirable temperature balance, cannot be solved even when the harvesting cycle start is controlled by a conventional control system. That is, when the harvesting thermostat is set to a higher temperature, the harvesting operation is protracted even during normal operation thus causing a loss in time. On the other hand, the pressure at the low pressure side during harvesting may be occasionally increased to an unusual value so that problems with respect to the durability of the compressor occur.
An advantageous feature of the rotary compressor is that it is lighter and more compact than a reciprocating compressor. However, its small thermal capacity resulting from its lightness proves to be a disadvantage when employed in an ice making machine using hot gas for defrosting. More specifically, during the defrosting cycle of the ice making machine, the ice formed by an evaporator is detached from the freezing mold by heating. During this defrosting cycle, the refrigerant gas is condensed and turned into a refrigerant liquid which is sucked in a large quantity into the compressor resulting in the cooling of the compressor. The rotary compressor having a smaller thermal capacity than the conventional reciprocating compressor is cooled more quickly than the reciprocating compressor. This indicates that the rotary compressor has an extremely low hot gas effectiveness as compared to the reciprocating compressor thus markedly affecting the defrosting ability at the lower temperature of the rotary compressor.